Iodine is an essential micronutrient needed for the biosynthesis of thyroid hormones, which are critical in the regulation of cellular metabolism, as well as in normal growth and mental development.1,2 In order to provide a consistent source of iodine to the population and prevent iodine deficiency disorders (IDDs), many countries have adopted table salt iodization as a public health policy. Although significant improvement has been achieved in the reduction of endemic goiter and cretism, other IDDs remain problematic with almost 2 billion people worldwide at risk for iodine insufficiency3 that is associated with impaired cognitive development in children, as well as weight gain, depression, thyroid disorders and cardiovascular diseases later in life.4-7 Several developed countries in which iodine deficiency was believed to be eradicated some decades ago, are facing mild iodine deficiency due to reduced intake of iodine-rich foods, increased consumption of non-iodized processed foods and/or exposure to iodide uptake inhibitors in the environment.8-11 For instance, about one third of pregnant women in the United States are marginally iodine deficient.5 Therefore, continuous monitoring of iodine status through iodide detection is an essential part of universal salt iodization programs10,12,13 with dietary iodide intake recommended to be 150 μg/day for adults and 250 μg/day for pregnant women.3 The best single measurement to evaluate median iodine intake in the population is via excreted urinary iodide, which represents more than 90% of the iodine recently ingested.2,3,14,15 For this reason, iodide concentrations from random spot urine samples are often used in epidemiological studies for iodine status assessment. However, simple, selective and cost-effective assays are needed to analyze sub-micromolar levels of urinary iodide for population health.16 
A kinetic spectrophotometric assay based on the classic Sandell-Kolthoff reaction is the most widely used method for urinary iodine determination, where iodide serves as a catalyst for the reduction of the yellow Ce(IV) to colorless Ce(III) in the presence of As(III).16,17 Although the assay is well established, a number of different protocols exist that are time-consuming, involve handling of toxic reagents, and require specially designed sealing cassettes for acid digestion in microplates to remove interferences in urine. Inductively coupled plasma-mass spectrometry (ICP-MS) is also used for urinary iodine determination. A simple dilution step with addition of an isotopic internal standard is used to minimize matrix effects while achieving low detection limits.17,18 However, the method is not specific for iodide, measuring instead total iodine in the sample, thus it is prone to bias due to iodine-containing compounds not completely bioavailable for thyroid uptake, such as iodinated drugs (e.g., amiodarone), radiologic contrast agents (e.g., iopamidol), and food additives (e.g., erythrosine). Also, large sample volumes (>200 μL) are often required for sample preparation when using pneumatic nebulizers with ICP-MS that limit the use of stored urine specimens in bio-repositories with finite volumes. Ion-exchange chromatography coupled to ICP-MS has been introduced for improved speciation of iodine, including iodide, iodate and iodine-containing analogues of tyrosine from edible seaweed.19 However, ICP-MS and other MS-based methods17,18 demand more costly infrastructure and operating costs that are not suitable for developing countries. Alternatively, capillary electrophoresis (CE) with UV detection has been reported for the analysis of iodide in urine20,21 and other complex sample matrices.21-25 Wide-bore capillaries21 and/or transient isotachophoretic (tITP) preconcentration20,22-24 are needed to attain sub-micromolar detection limits for assessment of iodine nutritional insufficiency (<0.79 μM or <100 μg/L) by the World Health Organization.3 However, the lack of rigorous method validation and long-term robustness studies has prevented the translation of CE-based assays for large-scale epidemiological or clinical studies.26 